The present invention relates to an improved video disc player. Specifically, the invention relates to a video disc player in which reproduced picture quality and reproduced sound quality are improved.
A prior art video disc player is constructed as shown in FIG. 1. A laser beam emanating from a laser tube 1 is focused onto the signal recorded surface of a disc 7 through a diverging lens 2, a beam splitter 3, a tangential mirror 5, a tracking mirror (not shown), and an objective lens 6. Reference numeral 8 designates a motor for turning the disc 7. Pits are formed on the surface of the disc along a spiral track. The length of the pits and the spacing between pits are representative of the recorded signal which is frequency modulated by the television video signal and the audio signal. The laser beam thus focused onto the surface of the disc is modulated by these pits. The beam reflected therefrom passes again through the objective lens 6 and the tangential mirror 5 and is applied to the beam splitter 3 for directing the reflected light toward a light receiving element 4. Changes in the amount of reflected light are converted into an electric signal by the light receiving element 4.
In this manner, the video signal and the audio signal recorded on the disc 7 are read. The video signal and the audio signal are multiplexed in the form of a frequency-modulated signals.
The output of the light receiving element 4 is amplified by an amplifier 9 and then applied to both a sound demodulator circuit 10 and a video demodulator circuit 12 to provide demodulated audio and video signals which are respectively fed through audio and video output terminals 11 and 13 to a TV monitor for reproduction.
For good video reproduction for color television, it is imperative to maintain the synchronizing signal frequency of the reproduced video signal at an accurate value and to suppress time base variations i.e., jitter, to a very small value, for example, no greater than 5 ns. For this purpose, it is necessary that the rotational speed of the disc 7 be precisely controlled. The speed of the motor 8 is controlled by a motor servo circuit while the deflection angle of the tangential mirror 5 is controlled by a tangential mirror servo circuit through a coil for driving the tangential mirror 5, which is an electromagnetically-controlled electromagnetic actuator.
First, the motor servo circuit will be described. A horizontal synchronizing signal obtained from a reproduced video signal and separated by a synchronizing signal separator circuit 14 is fed to a phase comparator 16 and is compared with a reference horizontal synchronizing signal which is produced in a reference synchronizing signal generator 15. The output of the phase comparator 16 is applied to a motor control circuit 17 for controlling the motor speed so that the two synchronizing signals are in phase. However, the circuit cannot adequately follow fast variations in the horizontal synchronizing signal due to the large moment of inertia of the disc 7, so that residual jitter at a higher frequency appears in the output. In order to eliminate such jitter, the tangential mirror 5, having a small amount of inertia, is moved in the direction of disc rotation in a direction to compensate for this jitter. (For clarity, the swinging direction of the tangential mirror is changed as shown in FIG. 1.)
Next, the tangential mirror servo circuit will be described. The reproduced video signal is fed to a burst gate circuit 18 where a burst signal containing reference color information is reproduced and then applied to a gate circuit 19. The gate circuit 19, which also receives a horizontal synchronizing signal reproduced and separated by the synchronizing signal separator circuit 14, operates to detect a specific rising or a falling edge of the reproduced burst signal which is generated at a predetermined time after the reproduced horizontal synchronizing signal. In other words, the gate circuit 19 produces a signal having a predetermined edge transition for every horizontal synchronizing signal reproduced, the timing edge being defined by the rising or falling pulse edge of the reproduced burst signal. In this case, the timing edge is preferably determined by the burst phase due to the phase accuracy of the burst signal, which 455/2 times higher than that of the horizontal synchronizing signal, because of the necessity for achieving a very high phase accuracy for proper reproduction of color signals.
The pulse signal thus obtained is fed to one terminal of the phase comparator 20 and is compared with the reference horizontal synchronizing signal which is applied to the other terminal thereof. Any phase difference thus detected is used to control the deflection angle of the tangential mirror 5 via the mirror control circuit 21. This results in the reproduction of stable color television at the video output terminal 13 without time base variations in synchronization with the reference sychronizing signal.
As indicated in FIG. 2A, however, no burst signal is present for a period of about 9H during the vertical blanking interval of the video signal, H representing one horizontal line scanning time which is 63.5 .mu.s for standard TV signals. Due to the absence of the burst signal, the gate circuit 19 may not operate properly during this interval with edge to the phase comparator 20.
Consequently, the phase comparator 20 may produce a faulty phase difference signal during the vertical retrace time, as indicated in FIG. 2B, and hence the tangential mirror 5 may tend to be deflected in the incorrect direction. When a burst signal A appears again following the vertical retrace time, the gate circuit 19 will again operate properly and the phase comparator 20 will then produce a proper phase difference signal which causes the tangential mirror 5 to return to the correct angular position. Thus, the mirror 5 is again deflected without any phase error after some transient time as indicated in FIG. 2B. This process is repeated every vertical retrace time.
It thus may be seen that errors in the deflection angle of the tangential mirror 5 can occur at the repetition rate of the vertical retrace time (60 Hz under the Specification of National Television Standard Committee). The errors in the mirror's angular position may cause undesired frequency variations in reproduced FM signals every vertical blanking interval. As a result, pulse noise may be produced in the output of the sound demodulator circuit 10 as indicated in FIG. 2C. This results in a lowering of the reproduced sound quality due to pulsive noise induced the output at the vertical retrace repetition rate as indicated in FIG. 3.
In summary, conventional video disc players as shown in FIG. 1 can reproduce color pictures of good stability, but have the disadvantage of poor sound reproduction due to inherent repeated pulsive noise.
An object of this invention is thus to provide a video disc player in which the above-noted drawback is eliminated and audio signals of high quality and video signals of high stability are produced without time base variations.